Islamic State has killed at least 41 people and injured more than 80 others in an attack on a Shia cultural centre and news agency that share a building in Kabul.

The bombings were the latest in a particularly bloody year for the Afghan capital, even by the standards of a country inured to decades of conflict.

The first explosion was detonated by a suicide bomber sitting among students at a lecture marking the 38th anniversary of the Russian invasion of Afghanistan. The death toll rose over the day as hospitals struggled to cope, and may climb further.

“The hall was packed with the people when there was a blast at the back,” said Ali Abbas Qul, a university student who had attended the lecture. He wept outside a hospital, in clothes still stained with the blood of two friends.

“People started running everywhere. Many lost consciousness. I lost my two friends and picked up their bodies. Many of the university students are still missing.”

Two more blasts outside the building targeted security and medical services, and people attempting to put out a fire started by the first bomb.

“I was trying to help people who were injured and killed in the first two blasts when the third explosion hit,” said Saed Qasem Rahmati, 35, a former employee at the cultural centre who rushed to help after the first bomb and was still in shock.

Many of the victims were badly burned, he said, and he feared that several friends might be among the unidentifiable bodies.

The explosions killed at least one journalist working for the Afghan Voice news agency, whose offices were on the floor above the cultural centre.

Ali Reza Ahmadi said he had leapt from his second-floor office after seeing flames coming from the building. “I jumped from the roof towards the basement, yelling at people to get water to put out the fire,” he told the AP.

Isis’s Amaaq news agency said the cultural centre had been targeted by Islamic State Khorasan Province (ISKP), the full name of the group’s Afghan affiliate, because it was funded by Iran and used to spread Shia beliefs.

It was the latest sectarian attack in a capital that had once been relatively immune to such violence. For all the civilian deaths in Kabul over four decades of civil war, until the rise of Isis very few could be chalked up to the Sunni-Shia tensions that have claimed so many lives from Iraq to Pakistan.

Now a group with a strong sectarian agenda appears to have outstripped the home-grown Taliban in inflicting violence on Kabul.

“[This is ISKP’s] 7th suicide bombing in the capital since 20 Oct, killing 130 people in total, making ISKP a larger threat in Kabul than Taliban for this period, at least,” Borhan Osman, a senior analyst for the International Crisis Group, tweeted.

The shift in attack rates from the group that has led the insurgency for nearly two decades to Isis, which is relatively new, was once “unimaginable”, Osman said.

The trend is worrying because very little is known about Isis operations in Kabul, Osman warned. Fighters in Afghanistan’s east, where Osama bin Laden once hid in the Tora Bora caves, have received far more press and military attention. “With [the] focus on Nangarhar, we know little about ISKP Kabul cell,” Osman wrote.

A senior Isis commander, speaking to the Guardian from an undisclosed location, said the group aimed to replace the Taliban, and would continue with its policy of sectarian attacks.

“Isis is continuously recruiting Taliban commanders and fighters, and deliberately challenging the Taliban’s dominance,” he said. “The Taliban in the past have negotiated with the US and the government in Kabul, diverting from the jihad. What they are doing is not Islamic jihad.”

“Beginning in 2016, a pattern of attacks against Shia worshippers emerged, mainly claimed by Islamic State Khorasan Province,” the UN said. Prior to that, the only major sectarian attack documented by the UN was a bombing at a religious festival in 2011, claimed by the Pakistani militant group Laskhar-e-Jhangvi.

The UN said many children were likely to be among those killed in Thursday’s bombing because families had gathered at the centre to mark the national day.

“I have little doubt that this attack deliberately targeted civilians,” said Toby Lanzer, the acting head of the mission in Afghanistan. “Today in Kabul we have witnessed another truly despicable crime in a year already marked by unspeakable atrocities.”